Gaza’s hospitals were struggling to cope with the flow of seriously injured people, doctors said, with video footage showing Palestinians desperately digging through the rubble of collapsed apartment blocks searching for survivors as ambulance sirens wailed in the distance.

Israel vowed to continue its bombardment and on Tuesday the government of Benjamin Netanyahu approved the call-up of an extra 40,000 reservists in preparation for a possible ground assault.

The IDF said militants from Gaza launched a barrage of more than 100 rockets on Tuesday night, with one long-range missile reaching as far as 99 kilometres into Israel. Some were intercepted by Israel’s Iron Dome missile system, others landed in fields or unpopulated areas.

There were no reports of casualties although the attacks sent terrified Israelis running for bomb shelters, with air raid sirens sounding in Jerusalem and Tel Aviv late on Tuesday.

Israeli forces say they prevented an attack by armed Palestinians who crossed from Gaza into Israel, killing five militants in the incident.

The IDF says it had hit “dozens of terror sites across the Gaza Strip, including concealed rocket launchers, launching infrastructures, a weapon storage facility, training bases, terror tunnels’ shafts and further targets”.

But the pictures coming out of Gaza told another story – that of civilians, including at least eight children according to Defence of Children International, dying along with the militants.

"We are preparing for a battle against Hamas which will not end within a few days," Israeli Defence Minister Moshe Yaalon said.

It is the heaviest fighting between Israel’s powerful military and militants in Gaza since the eight-day war in November 2012 in which 167 Palestinians, including 87 civilians, and four Israelis were killed.

Tensions  escalated sharply last week after the bodies of three Israeli teenagers who had been kidnapped in the occupied West Bank were found, and a Palestinian teenager was abducted and burned to death in what police believe was a revenge attack.

Washington condemned the rocket attacks from Gaza, while the EU’s foreign policy chief Catherine Ashton condemned both the “indiscriminate fire into Israel by militant groups in the Gaza Strip” and the “growing number of civilian casualties, reportedly among them children, caused by Israeli retaliatory fire”. She called on Israel and the Palestinians in Gaza "to do their utmost to achieve an immediate ceasefire”.

Palestinian media reported that Egyptian President Abdel Fattah al-Sisi had called Palestinian Authority President Mahmoud Abbas and pledged to work on a ceasefire, although there are no signs that hostilities will end any time soon.

Israeli forces killed six children when a missile struck the home of alleged Hamas activist Odeh Ahmad Kaware in the southern Gaza city of Khan Younis, Defence of Children International reported.

The five families living in the building evacuated after an Israeli aerial drone fired a warning missile, however a number of neighbours gathered on the roof in an effort to prevent the bombing.

Despite their presence, an Israeli air strike levelled the building, killing seven people including the six children and injuring 28 others.

Others killed in the air raids included four Hamas members who died in Gaza City when their car was struck, including Mohammed Shaaban, a senior militant.

Mr Netanyahu said “Hamas bears full responsibility for any harm that comes to Israeli and Palestinian civilians alike”: "In recent days, Hamas terrorists have fired hundreds of rockets at Israel's civilians. No other country lives under such a threat, and no country would accept such a threat.”

But some citizens in southern Israel begged their government to avoid further escalation, saying air strikes would only lead to more civilian deaths on both sides.

Julia Chaitin, who for 41 years has lived on Kibbutz Urim, 14 kilometres from the border with Gaza, described life as “insufferable and dangerous”.

“As a resident of the region, it is extremely important to me that people on both sides of the border will have peace and quiet and the real possibility to live lives of dignity without existential fear,” she wrote in a letter to the Israeli parliament this week.

Like many Israelis living close to the Gaza border, Ms Chaitan, who represents the Israeli non-governmental organisation Other Voice, hears both the Israeli air strikes on Gaza and the rockets fired on Israel from Gaza.

“It is all around me,” she said. “We have been trying to maintain contact with our friends in Gaza but it is difficult to keep communications going.”

On the other side of the border, Palestinians described the constant bombardment that shook buildings and shattered windows throughout Gaza – a 42-kilometre long strip that is under a sustained land, air and sea blockade from Israel that prevents most of Gaza's 1.7 million population from leaving.

Egypt has also kept its border with Gaza closed for most of the last year, although it announced it would open the Rafah Crossing to allow Gazans critically injured in the Israeli attacks to reach medical help.

"There has been no respite, not even for one hour," said Nasser, a student, speaking on the phone from his home in Gaza City. "They are hitting all over Gaza, there seems to be no area that's been spared."

“Shattered windows, terrified children, we are on the floor,” Dr Mona Qasim al-Farra said from her home near the Gaza Port.

“Right now it is Ramadan, the month of fasting, and an increasing number of families have difficulty to get basic food, survival is a constant fight. The military operation continues with threats of its expansion in the coming few days, and no news about any ceasefire.”

In a letter to the United Nations, Palestinian ambassador Riyad Mansour called on the UN Security Council to act against Israel’s bombardment.

“The intensification of Israel’s aggression against the 1.7 million Palestinians imprisoned in the Gaza Strip by Israel’s immoral blockade threatens to further destabilise the dangerous situation on the ground and fully ignite yet another round of deadly violence,” Dr Mansour wrote.